Imaging device, electronic apparatus, and imaging method

ABSTRACT

An imaging device includes a photodiode having a first electrode and a second electrode, a first transistor that controls electrical connection between the first electrode and a first wiring line through which first voltage is supplied, and a second transistor that controls electrical connection between the first electrode and a second wiring line through which second voltage different from the first voltage is supplied, and voltage at the second electrode is read with the first transistor and the second transistor turned off.

BACKGROUND

1. Technical Field

The present invention relates to an imaging device using photodiodes, anelectronic apparatus including the imaging device, and an imaging methodusing the imaging device.

2. Related Art

In a photodiode used in an imaging device, electric charge according tothe amount of received light is excited during a light exposure period.In the imaging device, however, after the light exposure but before thevoltage across the photodiode is read, leak current flows in thedirection from the N-type impurity layer of the photodiode toward theP-type impurity layer thereof (reverse direction), and the electriccharge excited in the photodiode therefore decreases. It is thereforeundesirably difficult to accurately detect the amount of light receivedwith the photodiode.

To solve the problem, JP-A-2006-216643 discloses an imaging deviceincluding a photodiode in which electric charge according to the amountof received light is excited, an amplification transistor that producesan imaging signal according to the voltage at the cathode of thephotodiode, and a leak current cancellation diode connected to thecathode of the photodiode. In the imaging device, in the period forwhich leak current flows from the cathode of the photodiode to the anodethereof, leak current flows into the cathode of the photodiode via theleak current cancellation diode to replenish the electric charge lostdue to the leak current in the photodiode.

In the technology disclosed in JP-A-2006-216643, the electric chargelost from the photodiode due to the leak current is replenished by theleak current from the leak current cancellation diode. It is thereforenecessary to set the magnitude of the leak current from the photodiodeto be equal to the magnitude of the leak current from the leak currentcancellation diode. The characteristics of the photodiode and the leakcurrent cancellation diode, however, vary product to product, and it istherefore difficult to set the magnitude of the leak current from thephotodiode to be equal to the magnitude of the leak current from theleak current cancellation diode. Therefore, according to the technologydisclosed in JP-A-2006-216643, it is difficult to acquire an imagingsignal accurately representing the amount of light received with thephotodiode.

SUMMARY

An advantage of some aspects of the invention is to accurately detectthe amount of light received with a photodiode.

An aspect of the invention provides an imaging device includingphotodiodes each having a first electrode and a second electrode, afirst transistor that controls electrical connection between the firstelectrode and a first wiring line through which first voltage issupplied, and a second transistor that controls electrical connectionbetween the first electrode and a second wiring line through whichsecond voltage different from the first voltage is supplied, and voltageat the second electrode is read with the first transistor and the secondtransistor turned off.

According to the aspect of the invention, the first transistor and thesecond transistor are turned off to disconnect a current path throughwhich leak current from the corresponding photodiode flows. Therefore,reading the voltage at the second electrode of the photodiode with thefirst transistor and the second transistor turned off allows accuratedetection of the amount of light received with the photodiode.

In a preferable aspect of the invention, a gate of the first transistoris connected to a rest line, a gate of the second transistor isconnected to a light exposure line, voltage that turns on the firsttransistor is outputted to the reset line and voltage that turns off thesecond transistor is outputted to the light exposure line to initializethe corresponding photodiode, voltage that turns off the firsttransistor is outputted to the reset line and voltage that turns on thesecond transistor is outputted to the light exposure line to exciteelectric charge according to an amount of received light in thecorresponding photodiode, and the voltage that turns off the firsttransistor is outputted to the reset line and the voltage that turns offthe second transistor is outputted to the light exposure line to readthe voltage at the second electrode.

In the configuration described above, the imaging device turns on or offeach of the first transistor and the second transistor to initialize thecorresponding photodiode, excite electric charge according to the amountof received light, and read the voltage at the second electrode. Theimaging device according to the preferable aspect of the invention cantherefore accurately detect the amount of light received with thephotodiode.

In a preferable aspect of the invention, the imaging device furtherincludes a capacitance element interposed between the second electrodeand the second wiring line.

In the configuration described above, since the capacitance element isinterposed between the second electrode and the second wiring line, theelectric charge excited in accordance with the amount of light receivedwith the corresponding photodiode can be determined from the electriccharge charged in the capacitance element. The amount of light receivedwith the photodiode can therefore be accurately detected.

In a preferable aspect of the invention, the imaging device furtherincludes a third transistor that amplifies the voltage at the secondelectrode and outputs the amplified voltage and a fourth transistor thatcontrols whether or not an output from the third transistor is allowedin accordance with voltage supplied from a selection line to a gate ofthe fourth transistor.

In the configuration described above, since the third transistoramplifies the voltage at the second electrode and outputs the amplifiedvoltage, the amount of light received with the corresponding photodiodecan be more accurately detected. Further, since the fourth transistorcontrols whether or not an output from the third transistor is allowedin accordance with voltage supplied from a selection line to the gate ofthe fourth transistor, the amount of light received with the photodiodecan be more accurately detected for each of the fourth transistorsconnected to the selection lines.

In the present application, the state in which an element A and anelement B are electrically connected to each other means that theelement A and the element B can be so set as to have the same logicalstate (voltage in design concept). Specifically, the state in which theelement A and the element B are electrically connected to each otherincludes not only a state in which the element A and the element B aredirectly connected to each other via a wiring line but also a state inwhich the element A and the element B are indirectly connected to eachother via a resistive element, a switching element, or any otherelectrical element. That is, the state of the element A and the elementB can be taken as the electrically connected state even in a case wherethe voltage across the element A and the voltage across the element Bslightly differ from each other but the voltages are handled as the samelogic in a circuit. Therefore, for example, in a state in which thefirst transistor is kept on, the state in which the first electrode ofthe corresponding photodiode is electrically connected to the firstwiring line is achieved.

The imaging device according to each of the aspects described above isused in a variety of electronic apparatus. A typical example of theelectronic apparatus is a video apparatus that captures images (stillimages and video images) of a subject, and the imaging device accordingto each of the aspects described above is further preferably used, forexample, in a biological information measurement apparatus that capturesan image of a vein in a biological body to measure biologicalinformation.

Another aspect of the invention provides an imaging method using animaging device including photodiodes each having a first electrode and asecond electrode, a first transistor that controls electrical connectionbetween the first electrode and a first wiring line through which firstvoltage is supplied, and a second transistor that controls electricalconnection between the first electrode and a second wiring line throughwhich second voltage different from the first voltage is supplied, themethod including reading voltage at the second electrode with the firsttransistor and the second transistor turned off.

In a preferable aspect of the invention, in the imaging device, a gateof the first transistor is connected to a rest line, and a gate of thesecond transistor is connected to a light exposure line, and the imagingmethod further includes outputting voltage that turns on the firsttransistor to the reset line and outputting voltage that turns off thesecond transistor to the light exposure line to initialize thecorresponding photodiode, outputting voltage that turns off the firsttransistor to the reset line and outputting voltage that turns on thesecond transistor to the light exposure line to excite electric chargeaccording to an amount of received light in the correspondingphotodiode, and outputting the voltage that turns off the firsttransistor to the reset line and outputting the voltage that turns offthe second transistor to the light exposure line to read the voltage atthe second electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described with reference to the accompanyingdrawings, wherein like numbers reference like elements.

FIG. 1 is a cross-sectional view of an imaging system including animaging device according to a first embodiment.

FIG. 2 shows an electrical configuration view of an imaging device.

FIG. 3 shows an electrical configuration view of a unit circuit.

FIG. 4 is a time chart showing the action of the imaging device.

FIG. 5 is a time chart showing the action of a column selection circuit.

FIG. 6 is a time chart showing the action of an imaging device accordingto a second embodiment.

DESCRIPTION OF EXEMPLARY EMBODIMENTS First Embodiment

A: Configuration

FIG. 1 shows the configuration view of an imaging system 100 includingan imaging device 18 according to a first embodiment of the invention.The imaging system 100 is an electronic apparatus that captures an imageof a subject 200 and includes an illuminator 12, a light collector 14, alight blocker 16, and the imaging device 18 according to the presentembodiment, as shown in FIG. 1. The illuminator 12, the light collector14, and the light blocker 16 are disposed on the imaging device 18. Thelight collector 14 is located between the illuminator 12 and the lightblocker 16, and the light blocker 16 is located between the lightcollector 14 and the imaging device 18. The light blocker 16 and theimaging device 18 are bonded to each other by using a light transmissiveadhesive 17. The imaging system 100 is used in a biological informationmeasurement apparatus that measures biological information, for example,the degree of oxygen saturation and the blood sugar level, and capturesan image of a vein in an examinee's hand or finger as an image of thesubject 200.

The illuminator 12 includes a light transmissive substrate 122 and aplurality of light emitting devices 124, as shown in FIG. 1. Theplurality of light emitting devices 124 are formed in the substrate 122,specifically, in the surface thereof facing away from the lightcollector 14 and emit illumination light of a specific wavelength withwhich the subject 200 is illuminated. For example, the plurality oflight emitting devices 124 irradiate the subject 200 with infrared light(having wavelength longer than or equal to 700 nm but shorter than orequal to 900 nm) as the illumination light. The infrared light passesthrough biological tissue of the subject 200 and is partially absorbedby reduced hemoglobin in the blood in the vein. Out of the infraredlight having passed through the biological tissue of the subject 200,infrared light that has not been absorbed by the reduced hemoglobin isreflected as imaged light off the biological tissue toward theilluminator 12. The subject 200 can instead be illuminated from the sideopposite the imaging device 18 with respect to the subject 200.

The light collector 14 is an element that collects the imaged lightcoming from the subject 200 and includes a light transmissive substrate142 and a plurality of lenses 144. Each of the plurality of lenses 144is formed on the substrate 142, specifically, on the surface thereoffacing the light blocker 16 and is a convex lens that collects theimaged light from the subject 200. The light blocker 16 includes a lighttransmissive substrate 162 and a light blocking layer 164. The lightblocking layer 164 is a light blocking thin film formed on the substrate162, specifically, on the surface thereof facing the imaging device 18.A plurality of openings 166, through which light fluxes having exitedout of the discrete lenses 144, are formed in the light blocking layer164.

The imaging device 18 captures the imaged light having originated fromthe subject 200 and passed through the illuminator 12, the lightcollector 14, and the light blocker 16. The imaging device 18 in thefirst embodiment is a solid-state imaging device in which a plurality ofunit circuits U (not shown in FIG. 1), which include photodiodes Dcorresponding to the discrete lenses 144, are arranged in a surface of asubstrate 182. The substrate 182 is made, for example, of asemiconductor material.

FIG. 2 shows an electrical configuration of the imaging device 18. Theimaging device 18 according to the first embodiment includes an imagingsection 20, a row selection circuit 32, a power source circuit 34, asignal output circuit 36, a reset signal generator 47, and a lightexposure signal generator 49, as shown in FIG. 2. The power sourcecircuit 34 produces high potential power source voltage Vdd and lowpotential power source voltage Vss (Vdd>Vss). In this example, the lowpotential power source voltage Vss is ground voltage. The high potentialpower source voltage Vdd is supplied to a first power source line 44,the reset signal generator 47, and the light exposure signal generator49, and the low potential power source voltage Vss is supplied to asecond power source line 42, the reset signal generator 47, and thelight exposure signal generator 49.

The row selection circuit 32, the power source circuit 34, the signaloutput circuit 36, the reset signal generator 47, and the light exposuresignal generator 49 in FIG. 2 function as a drive circuit for drivingthe imaging section 20 to detect the amount of light received with eachof the photodiodes D. The reset signal generator 47 includes a switchthat selects one of the high potential power source voltage Vdd and thelow potential power source voltage Vss supplied from the power sourcecircuit 34 and supplies the selected voltage as a reset signal Rst to areset line 46. The light exposure signal generator 49 includes a switchthat selects one of the high potential power source voltage Vdd and thelow potential power source voltage Vss supplied from the power sourcecircuit 34 and supplies the selected voltage as a light exposure signalExp to a light exposure line 48. Action of outputting the reset signalRst performed by the reset signal generator 47 and action of outputtingthe light exposure signal Exp performed by the light exposure signalgenerator 49 will be clearly described in detail in the description ofthe action of the imaging system 100.

The imaging section 20 has M selection lines 22, which extend in an Xdirection, and N detection lines 24, which intersect the selection lines22 extending in the X direction and extend in a Y direction (M and N arenatural numbers). The unit circuits U are disposed in the positionscorresponding to the intersections of the selection lines 22 and thedetection lines 24. That is, in the imaging section 20, a plurality ofunit circuits U are arranged in a matrix formed of M vertically arrangedrows and N horizontally arranged columns. Each of the unit circuits Uincludes a photodiode D (not shown in FIG. 2), in which electric chargeaccording to the amount of received light is excited. Each of the unitcircuits U receives the high potential power source voltage Vdd throughthe first power source light 44, the low potential power source voltageVss through the second power source light 42, the reset signal Rstthrough the reset line 46, and the light exposure signal Exp through thelight exposure line 48.

In the present embodiment, in accordance with occurrence of an imaginginstruction, initialization action of initializing electric chargeaccumulated in the photodiodes D in all the unit circuits U, lightexposure action of exciting electric charge according to the amounts oflight received by the photodiodes D in all the unit circuits U, andreadout action of reading the amount of the electric charge in thephotodiode D in each of the unit circuits U from the unit circuit U aresequentially performed.

The row selection circuit 32 and the signal output circuit 36 arecircuits that control the readout action of reading the amount of chargefrom each of the unit circuits U. In more detail, in each selectionperiod H, the row selection circuit 32 sequentially selects the row of aunit circuit U to be read, sets, among selection signals Y[m] (m=1 to M)outputted to the M selection lines 22, the level of a selection signalto be outputted to the selection line 22 in the selected row at anactive level (high level=Vdd in this example), and sets the levels ofthe selection signals to be outputted to the selection lines 22 in theother rows at an inactive level (low level=Vss in this example).

The signal output circuit 36 includes N transistors N4, which aredisposed in the respective columns of the unit circuits U in the imagingsection 20, a column selection circuit 38, and one output line 62. Thedrain of the transistor N4 in an arbitrary n column is connected to thedetection line 24 in the n-th column. The sources of the N transistorsN4 are connected to the common output line 62. The transistor N4 in then-th column is a switch that is interposed between the output line 62and the detection line 24 in the n-th column (n=1 to N) and controls theelectrical connection (conduction/isolation) between the output line 62and the detection line 24. That is, when the transistor N4 in the n-thcolumn is turned on, the detection line 24 in the n-th column iselectrically connected to the output line 62. The gates of the Ntransistors N4 are connected to the column selection circuit 38. Thecolumn selection circuit 38 sequentially selects the column of a unitcircuit U to be read in the period for which an arbitrary m-th row isselected by the row selection circuit 32 and outputs a control signalX[n] that turns on the transistor N4 in the selected column and turnsoff the transistors N4 in the other columns. The column selectioncircuit 38 sets the level of the control signal to be outputted to theselected column at an active level (high level=Vdd in this example) andsets the control signals to be outputted to the other columns at aninactive level (low level=Vss in this example).

FIG. 3 shows an electrical configuration view of a single unit circuit Ulocated in an arbitrary m row and n column in the imaging section 20.Each of the unit circuits U in the first embodiment includes thephotodiode D, transistors N1 to N3, a transistor P1, and a capacitiveelement, as shown in FIG. 3. The photodiode D is a light receivingdevice in which electric charge according to the amount of receivedlight is excited and has a stacked structure in which a photoelectricconversion layer is stacked between an anode E1, which is a firstelectrode, and a cathode E2, which is a second electrode. In thephotodiode D in the first embodiment, a chalcopyrite photoelectricconversion layer made, for example, of CuInSe₂ (CIS) or Cu(In,Ga)Se₂(CIGS) receives the imaged light from the subject 200. A chalcopyritephotoelectric conversion layer has high light reception sensitivity andcan therefore readily achieve highly sensitive imaging but has atendency toward large current leakage (dark current) at the time of nolight reception. The same tendency is observed, for example, in aconfiguration in which the photoelectric conversion layer of thephotodiode D is made of polycrystalline or amorphous silicon.

The anode E1 of the photodiode D is connected to the drain of thetransistor N3 and the drain of the transistor P1, as shown in FIG. 3.The source of the transistor N3 is connected to the second power sourceline 42, to which the low potential power source voltage Vss issupplied. The gate of the transistor N3 is connected to the lightexposure line 48, to which the light exposure signal Exp is supplied.The transistor N3 is turned on when the light exposure signal Expbecomes a high level (high potential power source voltage Vdd in thisexample) and electrically connects the anode E1 of the photodiode D tothe second power source line 42. The transistor N3, which is a secondtransistor, is thus interposed between the second power source line 42and the anode E1 of the photodiode D and functions as a switch thatcontrols the electrical connection (conduction/isolation) between thesecond power source line 42 and the anode E1 of the photodiode D.

The source of the transistor P1 is connected to the first power sourceline 44, to which the high potential power source voltage Vdd issupplied. The gate of the transistor P1 is connected to the reset line46, to which the reset signal Rst is supplied. The transistor P1 isturned on when the reset signal Rst becomes a low level (low potentialpower source voltage Vss in this example) and electrically connects theanode E1 of the photodiode D to the first power source line 44. Thetransistor P1, which is the first transistor, is interposed between thefirst power source line 44 and the anode E1 of the photodiode D andfunctions as a switch that controls the electrical connection(conduction/isolation) between the first power source line 44 and theanode E1 of the photodiode D.

The capacitive element C has one electrode connected to the cathode E2of the photodiode D and the other electrode connected to the secondpower source line 42. The capacitive element C retains the voltage atthe cathode E2 of the photodiode D.

The transistor N1 and N2 form an amplification circuit that outputsvoltage corresponding to the voltage at the cathode E2 of the photodiodeD to the detection line 24. In more detail, the transistor N2 has a gateconnected to the cathode E2 of the photodiode D, a source connected tothe detection line 24, and a drain connected to the source of thetransistor N1. The drain of the transistor N1 is connected to the firstpower source line 44, and transistor N1 is turned on when the selectionsignal Y[m], which is given to the gate of the transistor N1 viaselection line 22, becomes the high level (high potential power sourcevoltage Vdd in this example) and supplies the high potential powersource voltage Vdd to the drain of the transistor N2. The transistor N2thus functions as a source follower circuit and outputs the voltage atthe cathode E2 of the photodiode D from which threshold voltage of thetransistor N2 is subtracted to the detection line 24. The transistor N2,which is a third transistor, thus functions as an amplificationtransistor that amplifies the voltage at the cathode E2 of thephotodiode D and outputs the amplified voltage. The transistor N1, whichis a fourth transistor, functions as a switch that is interposed betweenthe transistor N2 and the first power source line 44 and controls theelectrical connection (conduction/isolation) between the transistor N2and the first power source line 44. The destination to which the drainof the transistor N1 is connected is not limited to the first powersource line 44, to which the high potential power source voltage Vdd issupplied. For example, the transistor N1 can be disposed between awiring line to which fixed voltage different from the high potentialpower source voltage Vdd is supplied and the drain of the transistor N2.The fixed voltage that is supplied to the wiring line in this case andis different from the high potential power source voltage Vdd may beproduced by an external power source or the power source circuit 34.

B: Action

The action of the imaging system 100 will next be described. FIG. 4 is atime chart showing the action of the imaging device 18 according to thepresent embodiment contained in the imaging system 100. In the imagingdevice 18, an initialization period (including standby period), a lightexposure period, and a readout period sequentially occur, for example,in response to an imaging instruction from a user. The initializationperiod is formed of a standby period immediately before the lightexposure period and a period that occurs before the standby period andhas a predetermined time length.

In each of the periods before the readout period, all the selectionsignals Y[1] to Y[M] are set at the low level, and the transistors N1 ofall the unit circuits U are off. Further, in each of the periods beforethe readout period, all the control signals X[1] to X[N] are set at thelow level.

In the initialization period but before the standby period, the lowpotential power source voltage Vss is supplied as the reset signal Rstto the reset line 46, and the low potential power source voltage Vss issupplied as the light exposure signal Exp to the light exposure line 48.As a result, in each of the unit circuits U of the imaging device 18,the transistor N3 is off, and the transistor P1 is on. Therefore, ineach of the unit circuits U, a current path including the first powersource line 44→the transistor P1→the photodiode D→the capacitive elementC→the second power source line 42 is formed, and current flows throughthe current path to initialize the photodiode D and charge thecapacitive element C.

In the standby period, the high potential power source voltage Vdd issupplied as the reset signal Rst to the reset line 46, and the lowpotential power source voltage Vss is supplied as the light exposuresignal Exp to the light exposure line 48. As a result, in each of theunit circuits U, the transistor N3 and the transistor P1 are off, sothat the current path described above is disconnected. The standbyperiod is provided to stabilize the voltage at the cathode E2 of thephotodiode D before the transition from the initialization period to thelight exposure period.

In the light exposure period, the high potential power source voltageVdd is supplied as the reset signal Rst to the reset line 46, and thehigh potential power source voltage Vdd is supplied as the lightexposure signal Exp to the light exposure line 48. As a result, in eachof the unit circuits U, the transistor N3 is on, and the transistor P1is off. In the light exposure period, leak current according to theamount of light received with each of the photodiodes D flows in thephotodiode D in the reverse direction. The leak current lowers thevoltage at the cathode E2 of the photodiode D, and the capacitiveelement C retains the voltage at the cathode E2.

When the readout period starts, the high potential power source voltageVdd is supplied as the reset signal Rst to the reset line 46, and thelow potential power source voltage Vss is supplied as the light exposuresignal Exp to the light exposure line 48. As a result, in each of theunit circuits U, the transistor N3 and the transistor P1 are off, sothat the current path through which the leak current from the photodiodeD flows is disconnected. Thereafter, in the readout period, in eachselection period H, the row selection circuit 32 sequentially selectsthe row of a unit circuit U to be read, sets, among the selectionsignals Y[m] to be outputted to the M selection lines 22, the level ofthe selection line 22 to be outputted to the selected row at the activelevel (high level=Vdd in this example), and sets the levels of theselection signals to be outputted to the selection lines 22 in the otherrows at the inactive level (low level=Vss in this example). When the rowselection circuit 32 supplies the selection signal Y[m] to the m-thselection line 22, the transistor N1 in the unit circuit U in the m-throw is turned on. As a result, in the unit circuit U in the m-th row,the transistor N2 functions as a source follower circuit, and thevoltage at the cathode E2 of the photodiode D from which the thresholdvoltage of the transistor N2 is subtracted is outputted to the detectionline 24.

In each selection period H, the column selection circuit 38 performsaction for capturing voltage from each of the N detection lines 24. FIG.5 is a time chart showing the action of the column selection circuit 38.In the selection period H in which an arbitrary row is selected by therow selection circuit 32, the column selection circuit 38 sequentiallyselects the column of a unit circuit U to be read and outputs a controlsignal X[n] that turns on the transistor N4 in the selected column andturns off the transistors N4 in the other columns, as shown in FIG. 5.The column selection circuit 38 sets the level of the control signal tobe outputted to the selected column at the active level (high level=Vddin this example) and sets the levels of the control signals to beoutputted to the other columns at the inactive level (low level=Vss inthis example). As a result, when the column selection circuit 38 selectsthe n-th column, the voltage outputted to the detection line 24 in then-th column is outputted to the output line 62 via the transistor N4 inthe n-th column and supplied to a processing circuit that is not shown.

As described above, according to the present embodiment, when the lightexposure period ends, in each of the unit circuits U, the transistor N3and the transistor P1 are turned off, so that the current path throughwhich leak current from the photodiode D flows is disconnected.Therefore, in the readout period, the amount of light received with thephotodiode D in each of the unit circuits U can be accurately detectedfrom the unit circuit U.

The capacitance of the capacitive element C is desirably 10 times theparasitic capacitance of each of the transistor N3 and the transistor P1or greater. The reason for this is as follows: In the readout period,since the transistor N3 and the transistor P1 are off, no current shouldflow in the photodiode D. In practice, however, a small amount ofcurrent actually flows through the transistor N3 and the transistor P1also in the readout period, and the electric charge accumulated in thecapacitive element C therefore flows into the transistor N3 and thetransistor P1 via the photodiode D. As a result, the voltage at thecathode E2 of the photodiode D decreases. However, when the capacitanceof the capacitive element C is 10 times the parasitic capacitance ofeach of the transistor N3 and the transistor P1 or greater, the amountof current that flows into the transistor N3 and the transistor P1 issmall in the readout period, whereby the resultant decrease in thevoltage at the cathode E2 of the photodiode D decreases. In a case wherethe photodiode D has the sufficiently large parasitic capacitancedescribed above, the capacitive element C may be omitted. In this case,the photodiode D plays the role of the capacitive element C.

Further, according to the present embodiment, since no leak current isallowed to flow in the photodiodes D when the light exposure periodends, the amount of light received with the photodiode D can beaccurately detected without the effect of the leak current even in asituation in which the light exposure period is so adjusted as to beshort.

Second Embodiment

An imaging device 18 according to a second embodiment of the inventionhas the same configuration as that of the imaging device 18 according tothe first embodiment. The imaging device 18 according to the presentembodiment differs from the imaging device 18 according to the firstembodiment only in terms of action thereof. FIG. 6 is a time chartshowing the action of the imaging device 18 according to the presentembodiment. In the present embodiment, the set of the initializationperiod (including standby period), the light exposure period, and thereadout period is repeated multiple times equal to the number of rows ofthe unit circuits U, while the row of a unit circuit U to be read isswitched to another, for example, in response of the user's imaginginstruction, as shown in FIG. 6. Further, in each readout period, therow selection circuit 32 sets, among the selection signals Y[1] to Y[M],the level of only the selection signal for selecting a row to be read atthe active level (high level=Vdd in this example) and sets the levels ofthe selection signals for selecting the other rows at the inactive level(low level=Vss in this example). The action of the column selectioncircuit 38 in the selection period H for which the row selection circuit32 selects an arbitrary m-th row is the same as the action in the firstembodiment (see FIG. 5).

The operation in which a row is sequentially selected from the first rowto the M-th row in each readout period and the electric charge is readfrom the unit circuit U in the selected row undesirably causes a longertime lag from the end timing of the light exposure period to the readouttiming in the unit circuit U in a row selected at a later point of time.Due to the time lag, the voltage of the cathode E2 of the photodiode Dlowers. It is therefore difficult to accurately detect the amount oflight received with the photodiode D depending on the length of the timelag. In the present embodiment, however, since the row selection circuit32 selects only one row in each readout period, the time lag from theend timing of the light exposure period to the readout timing does notincrease. Therefore, for example, even in a case where there are a largenumber of rows of the unit circuits U, the amount of light received withthe photodiode D can be accurately detected from each of the unitcircuits U.

The embodiments of the invention have been described above, andembodiments other than the above embodiments are conceivable, forexample, as follows.

(1) In each of the embodiments described above, the standby period isprovided in each initialization period, but the standby period may notbe provided. In this case, however, when the initialization period endsand the light exposure period starts, a delay between the rise of thelight exposure signal Exp from the low potential power source voltageVss to the high potential power source voltage Vdd and the rise of thereset signal Rst from the low potential power source voltage Vss to thehigh potential power source voltage Vdd causes a situation in which boththe transistor P1 and the transistor N3 are on to occur during thedelay, possibly resulting in short-circuit current flowing from thefirst power source line 44 to the second power source line 42 or viceversa. Therefore, in the case where no standby period is provided, thecircuit design needs to be so made that there is no delay between therise of the light exposure signal Exp from the low potential powersource voltage Vss to the high potential power source voltage Vdd andthe rise of the reset signal Rst from the low potential power sourcevoltage Vss to the high potential power source voltage Vdd at the startof the light exposure period.

(2) In each of the embodiments described above, no transistor N2 may bepresent. In this case, the source of the transistor N1 is connected tothe detection line 24, and the drain of the transistor N1 is connectedto the cathode E2 of the photodiode D. Therefore, in this case, thevoltage of the cathode E2 of the photodiode D is outputted to thedetection line 24.

(3) In each of the embodiments described above, no transistor N1 may bepresent. In this case, each of the unit circuits U heeds to be providedwith a dedicated detection line 24 and transistor N4.

(4) In each of the embodiments described above, the capacitance elementC has one electrode connected to the cathode E2 of the photodiode D andthe other electrode connected to the second power source line 42.Instead, the one electrode may be connected to the cathode E2 of thephotodiode D, and the other electrode may be connected to an externalpower source that supplies fixed voltage different from the highpotential power source voltage Vdd. The voltage supplied by the externalpower source may be voltage equal to the low potential power sourcevoltage Vss. Also in this case, in each initialization period but beforethe standby period, the transistor N3 is off, and the transistor P1 ison, so that the photodiode D is so initialized as to have predeterminedpotential. Thereafter, the light exposure period elapses, and thevoltage according to the amount of light received in the light exposureperiod can therefore be read from the photodiode D. Further, theexternal power source may be the power source circuit 34. For example,the power source circuit 34 may produce not only the high potentialpower source voltage Vdd and the low potential power source voltage Vssbut also intermediate voltage Vce representing a voltage valueintermediate between the high potential power source voltage Vdd and thelow potential power source voltage Vss, and the capacitance element Cmay have one electrode connected to the cathode E2 of the photodiode Dand the other electrode receiving the intermediate voltage Vice from thepower source circuit 34. Also in this case, in each initializationperiod but before the standby period, the transistor N3 is off, and thetransistor P1 is on, and the photodiode D is so initialized as to havepredetermined potential. Thereafter, the light exposure period elapses,and the voltage according to the amount of light received in the lightexposure period can therefore be read from the photodiode D.

(5) In each of the embodiments described above, the transistor N3 is anN-channel transistor but may instead be a P-channel transistor, which isa transistor having a different conductivity type. In this case,however, in each initialization period (including standby period) andthe readout period, the high potential power source voltage Vdd issupplied as the light exposure signal Exp to the light exposure line 48,and in the following light exposure period, the low potential powersource voltage Vss is supplied as the light exposure signal Exp to thelight exposure line 48. In this case, the reset signal Rst is the sameas the reset signal Rst in each of the embodiments described above.Further, in each of the embodiments described above, the transistor P1is a P-channel transistor but may instead be an N-channel transistor,which is a transistor having a different conductivity type. In thiscase, however, in each initialization period but before the standbyperiod, the high potential power source voltage Vdd is supplied as thereset signal Rst to the reset line 46, and in the following standbyperiod, light exposure period, and readout period, the low potentialpower source voltage Vss is supplied as the reset signal Rst to thereset line 46. In this case, the light exposure signal Exp is the sameas the light exposure signal Exp in each of the embodiments describedabove.

(6) In each of the embodiments described above, the transistor P1 isdisposed between the first power source line 44, to which the highpotential power source voltage Vdd is supplied, and the photodiode D,but the destination to which the photodiode D is connected via thetransistor P1 is not limited to the first power source line 44. Forexample, the transistor P1 can be disposed between a wiring line towhich fixed voltage different from the high potential power sourcevoltage Vdd is supplied and the photodiode D. Similarly, the destinationto which the photodiode D is connected via the transistor N3 is notlimited to the second power source line 42. For example, the transistorN3 can be disposed between a wiring line to which fixed voltagedifferent from the low potential power source voltage Vss is suppliedand the photodiode D. As will be understood from the above description,the first power source line 44, to which the high potential power sourcevoltage Vdd is supplied, is an example of a first wiring line to whichfirst voltage is supplied, and the second power source line 42, to whichthe low potential power source voltage Vss is supplied, is an example ofa second wiring line to which second voltage different from the firstvoltage is supplied.

(7) The row selection circuit 32, the column selection circuit 38, andthe signal output circuit 36 can be embodied in the form of anintegrated circuit (IC chip) mounted on the substrate 182 or can insteadbe each embodied, for example, in the form of a transistor directlyformed in the substrate 182.

(8) Biological information that can be measured by the biologicalinformation measurement apparatus using the imaging system 100 in eachof the embodiments described above is not limited to the informationdescribed in each of the embodiments described above (degree of oxygensaturation and blood sugar level). For example, the imaging system 100according to each of the embodiments described above can also be used tomeasure the concentration of alcohol, a cholesterol value, and otherpieces of biological information in the blood flowing through a vein.

(9) The electronic apparatus using the imaging device 18 (imaging system100) shown in each of the embodiments described above is not limited tothe biological information measurement apparatus shown in theembodiments described above. For example, the imaging device 18 shown inthe embodiments described above can be used in a variety of electronicapparatus, such as a digital still camera and a digital camcorder (videocamera) that capture still images, video images, and other types ofimages.

The entire disclosure of Japanese Patent Application No. 2015-227721filed on Nov. 20, 2015 is hereby incorporated herein by reference.

What is claimed is:
 1. An imaging device comprising: photodiodes eachhaving a first electrode and a second electrode; a first transistor thatcontrols electrical connection between the first electrode and a firstwiring line through which first voltage is supplied; and a secondtransistor that controls electrical connection between the firstelectrode and a second wiring line through which a second voltagedifferent from the first voltage is supplied, wherein voltage at thesecond electrode is read with the first transistor and the secondtransistor turned off.
 2. The imaging device according to claim 1,wherein a gate of the first transistor is connected to a reset line, agate of the second transistor is connected to a light exposure line,voltage that turns on the first transistor is outputted to the resetline and voltage that turns off the second transistor is outputted tothe light exposure line to initialize the corresponding photodiode,voltage that turns off the first transistor is outputted to the resetline and voltage that turns on the second transistor is outputted to thelight exposure line to excite electric charge according to an amount ofreceived light in the corresponding photodiode, and the voltage thatturns off the first transistor is outputted to the reset line and thevoltage that turns off the second transistor is outputted to the lightexposure line to read the voltage at the second electrode.
 3. Theimaging device according to claim 1, further comprising a capacitanceelement interposed between the second electrode and the second wiringline.
 4. The imaging device according to claim 1, further comprising: athird transistor that amplifies the voltage at the second electrode andoutputs the amplified voltage; and a fourth transistor that controlswhether or not an output from the third transistor is allowed inaccordance with voltage supplied from a selection line to a gate of thefourth transistor.
 5. A biological information measurement apparatuscomprising the imaging device according to claim
 1. 6. A biologicalinformation measurement apparatus comprising the imaging deviceaccording to claim
 2. 7. A biological information measurement apparatuscomprising the imaging device according to claim
 3. 8. A biologicalinformation measurement apparatus comprising the imaging deviceaccording to claim
 4. 9. An electronic apparatus comprising the imagingdevice according to claim
 1. 10. An electronic apparatus comprising theimaging device according to claim
 2. 11. An electronic apparatuscomprising the imaging device according to claim
 3. 12. An electronicapparatus comprising the imaging device according to claim
 4. 13. Animaging method using an imaging device including photodiodes each havinga first electrode and a second electrode, a first transistor thatcontrols electrical connection between the first electrode and a firstwiring line through which first voltage is supplied, and a secondtransistor that controls electrical connection between the firstelectrode and a second wiring line through which second voltagedifferent from the first voltage is supplied, the method comprisingreading voltage at the second electrode with the first transistor andthe second transistor turned off.
 14. The imaging method according toclaim 13, wherein in the imaging device, a gate of the first transistoris connected to a rest line, and a gate of the second transistor isconnected to a light exposure line, and the imaging method furtherincludes outputting voltage that turns on the first transistor to thereset line and outputting voltage that turns off the second transistorto the light exposure line to initialize the corresponding photodiode,outputting voltage that turns off the first transistor to the reset lineand outputting voltage that turns on the second transistor to the lightexposure line to excite electric charge according to an amount ofreceived light in the corresponding photodiode, and outputting thevoltage that turns off the first transistor to the reset line andoutputting the voltage that turns off the second transistor to the lightexposure line to read the voltage at the second electrode.